1. Field of the Invention
The present invention concerns a reliable, fast-acting, web wrap detection system for use with a printing press or the like which uses optical sensors to sense an increase in reflectance in the vicinity of a blanket cylinder and to stop printing press operation in response, such being indicative that the higher reflectance printing web has begun to wrap the cylinder as a result of web breakage.
2. Description of the Prior Art
In prior art printing presses or other devices which process an elongated fast moving web, the web travels through the device between pairs of opposed rotating rollers or cylinders. In these prior art devices, web breakage is typically detected by a means of so-called "electric eyes" mounted with a line of sight transverse to the path of travel of the web.
These prior art web wrap detection devices present a problem, however, in that they do not sense the web breakage until the trailing edge of the forward portion of the broken web passes by the next downstream electric eye. By the time the web breakage is detected, the upstream portion of the web may have already wrapped around one of the upstream cylinders or rollers. This results in additional down time to remove the wrapped web from the cylinders and, in some cases, may result in damage to the drive train. This problem is typically most severe in a printing press if the web breakage occurs downstream of the last electric eye just before the web enters the so-called "printing oven". When this occurs, the web may wrap itself many times around the last printing blanket cylinder before the break is detected as it exits the oven.